Show documentation for a class if the constructor has no docs

#KT-17926 Fixed

idea/src/org/jetbrains/kotlin/idea/KotlinDocumentationProvider.kt

diff --git a/idea/src/org/jetbrains/kotlin/idea/KotlinDocumentationProvider.kt b/idea/src/org/jetbrains/kotlin/idea/KotlinDocumentationProvider.kt
index 58547a7..d6fb301 100644
--- a/idea/src/org/jetbrains/kotlin/idea/KotlinDocumentationProvider.kt
+++ b/idea/src/org/jetbrains/kotlin/idea/KotlinDocumentationProvider.kt
@@ -6,7 +6,6 @@
 package org.jetbrains.kotlin.idea
 
 import com.google.common.html.HtmlEscapers
-import com.intellij.codeInsight.documentation.DocumentationManager
 import com.intellij.codeInsight.documentation.DocumentationManagerUtil
 import com.intellij.codeInsight.javadoc.JavaDocInfoGeneratorFactory
 import com.intellij.lang.documentation.AbstractDocumentationProvider
@@ -33,7 +32,6 @@
 import org.jetbrains.kotlin.idea.kdoc.KDocRenderer.appendKDocContent
 import org.jetbrains.kotlin.idea.kdoc.KDocRenderer.appendKDocSections
 import org.jetbrains.kotlin.idea.kdoc.KDocTemplate.DescriptionBodyTemplate
-import org.jetbrains.kotlin.idea.references.KtDescriptorsBasedReference
 import org.jetbrains.kotlin.idea.references.resolveToDescriptors
 import org.jetbrains.kotlin.idea.references.mainReference
 import org.jetbrains.kotlin.idea.resolve.ResolutionFacade
@@ -182,7 +180,10 @@
             defaultParameterValueRenderer = { (it.source.getPsi() as? KtParameter)?.defaultValue?.text ?: "..." }
         }
 
-        fun StringBuilder.renderKDoc(contentTag: KDocTag, sections: List<KDocSection>) {
+        private fun StringBuilder.renderKDoc(
+            contentTag: KDocTag,
+            sections: List<KDocSection> = if (contentTag is KDocSection) listOf(contentTag) else emptyList()
+        ) {
             insert(DescriptionBodyTemplate.Kotlin()) {
                 content {
                     appendKDocContent(contentTag)
@@ -215,7 +216,7 @@
                     }
                     if (!quickNavigation && kdoc != null) {
                         description {
-                            renderKDoc(kdoc.getDefaultSection(), listOf(kdoc.getDefaultSection()))
+                            renderKDoc(kdoc.getDefaultSection())
                         }
                     }
                 }
@@ -312,10 +313,9 @@
                         return mixKotlinToJava(declarationDescriptor, element, originalElement)
                     }
                 }
-            } else {
-                // This element was resolved to non-kotlin element, it will be rendered with own provider
             }
 
+            // This element was resolved to non-kotlin element, it will be rendered with own provider
             return null
         }
 
@@ -384,11 +384,22 @@
 
                 if (!quickNavigation) {
                     description {
-                        val comment = declarationDescriptor.findKDoc { DescriptorToSourceUtilsIde.getAnyDeclaration(ktElement.project, it) }
-                        if (comment != null) {
-                            val sectionList = if (comment is KDocSection) listOf(comment) else emptyList()
-                            renderKDoc(comment, sectionList)
-                        } else if (declarationDescriptor is CallableDescriptor) { // If we couldn't find KDoc, try to find javadoc in one of super's
+                        declarationDescriptor.findKDoc { DescriptorToSourceUtilsIde.getAnyDeclaration(ktElement.project, it) }?.let {
+                            renderKDoc(it)
+                            return@description
+                        }
+                        if (declarationDescriptor is ClassConstructorDescriptor && !declarationDescriptor.isPrimary) {
+                            declarationDescriptor.constructedClass.findKDoc {
+                                DescriptorToSourceUtilsIde.getAnyDeclaration(
+                                    ktElement.project,
+                                    it
+                                )
+                            }?.let {
+                                renderKDoc(it)
+                                return@description
+                            }
+                        }
+                        if (declarationDescriptor is CallableDescriptor) { // If we couldn't find KDoc, try to find javadoc in one of super's
                             insert(DescriptionBodyTemplate.FromJava()) {
                                 body = extractJavaDescription(declarationDescriptor)
                             }

idea/testData/editor/quickDoc/OnEmptySecondaryConstructor.kt

diff --git a/idea/testData/editor/quickDoc/OnEmptySecondaryConstructor.kt b/idea/testData/editor/quickDoc/OnEmptySecondaryConstructor.kt
new file mode 100644
index 0000000..f957bca
--- /dev/null
+++ b/idea/testData/editor/quickDoc/OnEmptySecondaryConstructor.kt
@@ -0,0 +1,17 @@
+/**
+Documentation is here.
+ */
+class Foo {
+    private var x: Int = 0
+
+    constructor(x: Int) {
+        this.x = x
+    } //secondary constructor without documentation
+}
+
+fun test() {
+    val f = Foo<caret>(10)
+}
+//INFO: <div class='definition'><pre><a href="psi_element://Foo"><code>Foo</code></a><br>public constructor <b>Foo</b>(
+//INFO:     x: Int
+//INFO: )</pre></div><div class='content'><p>Documentation is here.</p></div><table class='sections'></table>

idea/tests/org/jetbrains/kotlin/idea/editor/quickDoc/QuickDocProviderTestGenerated.java

diff --git a/idea/tests/org/jetbrains/kotlin/idea/editor/quickDoc/QuickDocProviderTestGenerated.java b/idea/tests/org/jetbrains/kotlin/idea/editor/quickDoc/QuickDocProviderTestGenerated.java
index acc30f7..6447454 100644
--- a/idea/tests/org/jetbrains/kotlin/idea/editor/quickDoc/QuickDocProviderTestGenerated.java
+++ b/idea/tests/org/jetbrains/kotlin/idea/editor/quickDoc/QuickDocProviderTestGenerated.java
@@ -158,6 +158,11 @@
         runTest("idea/testData/editor/quickDoc/OnClassDeclarationWithNoPackage.kt");
     }
 
+    @TestMetadata("OnEmptySecondaryConstructor.kt")
+    public void testOnEmptySecondaryConstructor() throws Exception {
+        runTest("idea/testData/editor/quickDoc/OnEmptySecondaryConstructor.kt");
+    }
+
     @TestMetadata("OnEnumClassReference.kt")
     public void testOnEnumClassReference() throws Exception {
         runTest("idea/testData/editor/quickDoc/OnEnumClassReference.kt");